Multi-strike process for bonding

1. A method comprising: performing a first strike process to strike a metal bump of a first package component against a metal pad of a second package component, wherein a first one of the metal bump and the metal pad comprises copper, and a second one of the metal bump and the metal pad comprises aluminum; performing a second strike process to strike the metal bump against the metal pad; and performing an annealing to bond the metal bump on the metal pad.

2. The method of claim 1 further comprising a third strike process after the first strike process and before the second strike process, wherein the third strike process comprises striking the metal bump against the metal pad.

3. The method of claim 1 , wherein before the first strike process, both a first oxide layer at a surface of the metal bump and a second oxide layer at a surface of the metal pad are continuous oxide layers, with the first oxide layer striking the second oxide layer in the first strike process.

4. The method of claim 3 , wherein both the first oxide layer and the second oxide layer break into pieces in the first strike process and the second strike process.

5. The method of claim 1 , wherein the first strike process is performed with the first package component moving from a position spaced apart from the second package component to impact the second package component.

6. The method of claim 1 , wherein after each of the first strike process and the second strike process, the metal bump extends into the metal pad.

7. The method of claim 1 , wherein during the annealing, the metal bump remains extending into the metal pad.

8. The method of claim 1 , wherein after the second strike process, the metal bump remains to be in contact with the metal pad until the annealing is finished.

9. A method comprising: raising a first package component over and spaced apart from a second package component; performing a first strike process to strike the first package component against the second package component; after the first strike process, raising the first package component over and spaced apart from the second package component; performing a second strike process to strike the first package component against the second package component; and annealing the first package component and the second package component to bond the first package component to the second package component.

10. The method of claim 9 , wherein a first one of the first package component and the second package component comprises a protruding metal bump, and a second one of the first package component and the second package component comprises a recessed metal pad, and wherein in the first strike process and the second strike process, the protruding metal bump strikes the recessed metal pad.

11. The method of claim 10 , wherein a first oxide layer on a surface of the protruding metal bump and a second oxide layer on a surface of the recessed metal pad are broken into pieces by the first strike process and the second strike process, and after the second strike process, an un-oxidized surface of the protruding metal bump is in physical contact with an un-oxidized surface of the recessed metal pad.

12. The method of claim 10 , wherein the second strike process results in the protruding metal bump to extend into the recessed metal pad, and wherein in an entire duration between the second strike process and the annealing, the protruding metal bump remains extending into the recessed metal pad.

13. The method of claim 10 , wherein an edge portion of the recessed metal pad is covered by a passivation layer, and in the first strike process, the protruding metal bump extends into an opening in the passivation layer.

14. The method of claim 9 , wherein in both the first strike process and the second strike process, the first package component impacts the second package component at a speed higher than about 100 μm/second.

15. A method comprising: performing a first strike process to strike a first package component against a second package component, wherein the first strike process results in a copper oxide layer on a surface of a copper pillar in the first package component to be broken, and an aluminum oxide layer on a surface of an aluminum pad in the second package component to be broken; separating the first package component from the second package component; after the first package component and the second package component are separated from each other, contacting the copper pillar with the aluminum pad; and with the copper pillar being in contact with the aluminum pad, annealing the first package component and the second package component to bond the first package component to the second package component.

16. The method of claim 15 , wherein the contacting the copper pillar with the aluminum pad comprises a second strike process for striking the first package component against the second package component.

17. The method of claim 16 further comprising a third strike process for striking the first package component against the second package component, wherein the third strike process is between the first strike process and the second strike process.

18. The method of claim 15 , wherein at a time the annealing is started, an un-oxidized portion of the copper pillar is in physical contact with an un-oxidized portion of the aluminum pad.

19. The method of claim 15 , wherein during the annealing, the copper pillar extends into the aluminum pad.

20. The method of claim 15 , wherein in the first strike process, the first package component impacts the second package component at a speed higher than about 100 μm/second.